The present invention relates to methods of manufacturing thin film photoelectric converters and plasma CVD apparatuses used for the methods, and more particularly, to reduction in the cost of silicon-based (silicon or silicon alloy) photoelectric converters and improvements in the performance thereof. Note that in this specification, the terms, xe2x80x9cpolycrystallinexe2x80x9d, xe2x80x9cmicrocrystallinexe2x80x9d and xe2x80x9ccrystallinexe2x80x9d refer not only to complete crystalline states, but also to partially amorphous states.
Typical thin film photoelectric converters include amorphous silicon-based solar cells. Since amorphous photoelectric conversion materials are formed by plasma CVD, normally at low temperatures around 200xc2x0 C., the materials can be formed on inexpensive substrates such as glass, stainless steel, and organic films and this is why they are believed to be promising as materials for manufacturing low cost photoelectric converters. Furthermore, since in amorphous silicon, the absorption coefficient is large in the visible light region, short-circuit current of 15 mA/cm2 or higher has been achieved in a solar cell using an amorphous photoelectric conversion layer having a film thickness of 500 nm or less.
The amorphous silicon-based materials, however, suffer from so-called Stebler-Wronskey effect, i.e. deterioration in the photoelectric conversion characteristic by long-term light irradiation, and the effective sensitivity wavelength region ranges up to about 800 nm. Therefore, in a photoelectric converter using an amorphous silicon-based material, the reliability and performance improvement is limited, and essential advantages of the material that it allows for flexibility in selecting a substrate or applicability in low cost process are not fully taken advantage of.
Meanwhile, in recent years, much energy has been devoted to development of photoelectric converters using a thin film including a crystalline silicon layer such as polycrystalline silicon and microcrystalline silicon. The development is an attempt to compatibly achieve reduction in the cost of a photoelectric converter and improvement in the performance by forming a high quality, crystalline silicon thin film on an inexpensive substrate in a low temperature process and its application to various photoelectric converters such as photo sensor in addition to solar cells is expected.
There are methods of forming such crystalline silicon thin films including directly depositing a film on a substrate by CVD or sputtering and depositing an amorphous film on a substrate in a similar process, followed by thermal anneal or laser anneal for crystallization. Any of these methods requests that the process must be carried out at a temperature of 550xc2x0 C. or less in order to use an inexpensive substrate as described above.
Among such processes, it is expected that the method of directly depositing a crystalline silicon thin film by plasma CVD can most readily achieve reduction in the process temperature and increase in the area of the film, and that a high quality film can be relatively easily obtained. If a polycrystalline silicon thin film is obtained by this method, a high quality, crystalline silicon thin film is formed on a substrate by some process, and then, using the film as a seed layer or a crystallization control layer, a film may be formed thereon such that a high quality, polycrystalline silicon thin film can be formed at a relatively low temperature.
Meanwhile, there is a well known method of forming a film, using a silane-based material gas diluted 10 times or more with hydrogen, by setting the pressure in a plasma reaction chamber in the range from 10 mTorr to 1 Torr to obtain a microcrystalline silicon thin film. In this method, a silicon thin film can be readily formed into microcrystal at a temperature around 200xc2x0 C. For example, a photoelectric converter including a photoelectric conversion unit formed by a pin junction of microcrystalline silicon is disclosed in Appl. Phys. Lett., Vol. 65, 1994, p. 860. The photoelectric conversion unit includes a p-type semiconductor layer, an i-type semiconductor layer which is a photoelectric conversion layer and an n-type semiconductor layer, simply deposited sequentially by plasma CVD, and all the semiconductor layers are of microcrystalline silicon. However, the deposition speed is less than 10 nm/min in the thickness-wise direction by conventional methods and under conventional conditions and too low for obtaining a high quality, crystalline silicon film and a high performance, silicon-based thin film photoelectric converter, and the speed is not more than the case of forming amorphous silicon films.
Meanwhile, an example of a silicon film formed at a relatively high pressure of 5 Torr by low temperature plasma CVD is disclosed by Japanese Patent Laying-Open No. 4-137725. In this example, however, the silicon thin film is directly deposited on a substrate of glass or the like and is presented simply as an example in comparison to the invention disclosed by Japanese Patent Laying-Open No. 4-137725, and the quality of the film is too low to be applied to photoelectric converters.
In general, if the pressure condition is raised in plasma CVD, a considerable amount of powdery product or dust is generated in the plasma reaction chamber, in which case such dust is likely to fly and come into the deposited film and generate pin holes in the film. In order to reduce such deterioration in the quality of the film, the reaction chamber must be frequently cleaned inside. Particularly when a film is formed at a low temperature of 550xc2x0 C. or less and the pressure in the reaction chamber is raised, such disadvantages can be noticed. In the manufacture of a photoelectric converter such as a solar cell, a thin film having a large area must be deposited, which could lower the yield and increase the labor and cost for maintaining the film forming device.
Therefore, when a thin film photoelectric converter is manufactured by plasma CVD, conventional methods have employed a pressure condition of 1 Torr or less as described above.
A polycrystalline photoelectric converter including a crystalline silicon-based thin film photoelectric conversion layer as described above suffers from the following disadvantage. More specifically, when used as a photoelectric conversion layer for a solar cell, the film thickness must be at least several xcexcm to several tens Ktm so that the film is allowed to absorb enough sunlight in view of the absorption coefficient of crystalline silicon, whether it is polycrystalline silicon or partly amorphous microcrystalline silicon. This thickness is larger than that of an amorphous silicon photoelectric conversion layer by about one or two digits.
Meanwhile, by conventional methods, the forming speed of a film is not more than that of an amorphous silicon film, as low as about 10 nm/min for example if various condition parameters including temperature, pressure in the reaction chamber, high frequency power and gas flow ratio are considered for obtaining a high quality, crystalline silicon-based thin film at a low temperature by plasma CVD. Stated differently, the crystalline silicon thin film photoelectric conversion layer requires time for forming several to several ten times as long as that of an amorphous silicon photoelectric conversion layer, which impedes the throughput in the manufacturing process of photoelectric converters from increasing and the cost from being reduced.
Conventional apparatuses to produce solar cells include those of the inline type according to which a plurality of film deposition chambers are linearly coupled as shown the block diagram in FIG. 6 and those of the multi-chamber type according to which a plurality of chambers are arranged around an intermediate chamber as shown in the block diagram in FIG. 7. Note that for an amorphous silicon solar cell, there is also known a simple method called xe2x80x9csingle chamber methodxe2x80x9d, according to which all the semiconductor layers are formed in the same chamber. However, in order to prevent conductivity determining impurity atoms which are doped into p-type layers and n-type layers from coming into different kinds of semiconductor layers, the gas in the chamber must be completely replaced for example by gas replacement for 1 hour using a purge gas such as hydrogen before forming these semiconductor layers. Even such gas replacement process could not secure high performance of an amorphous silicon solar cell, the single chamber method is no longer used other than for experimental purposes. Furthermore, in the manufacture of an amorphous silicon solar cell, a layer of one conductivity type, a photoelectric conversion layer and a layer of the opposite conductivity type must be sequentially formed in a vacuum process without ever being exposed to the atmosphere in this method, and therefore the inline method or multi-chamber method is employed in industrial applications.
For example, in an nip type solar cell in which an n-layer, an i-layer and a p-layer are sequentially deposited from the side of the substrate, according to the inline method as shown in FIG. 6, an n-layer deposition chamber 3n to form the n-layer, 6 i-layer deposition chambers 3l1 to 3l6 to form a photoelectric conversion layer, and a p-layer deposition layer 3p to form the p-layer are continuously coupled. In this case, the n and p layers are thinner than the i-layer and require significantly shorter time to form than the i layer, a plurality of such i-layer deposition chambers are usually coupled to raise the productivity, and until the time for forming the n and p layers becomes the rate determining factor, the productivity improves as the number of i-layer deposition chambers increases. In this inline method, however, the plurality of i-layer deposition chambers thus provided require maintenance most, and therefore the entire production line must be stopped for maintenance of even a single i-layer deposition chamber.
Meanwhile, in the multi-chamber method shown in FIG. 7, a substrate on which films are to be deposited is moved through an intermediate chamber 4m to each of film deposition chambers 4n, 4i1 to 4l4 and 4p. Since a movable partition to keep airtightness is provided between each chamber and the intermediate chamber, a trouble caused in a certain chamber does not obstruct the use of the other chambers, and therefore the whole production line does not have to be stopped for the trouble. The producing apparatus according to this multi-chamber method however has a complicated an d therefore expensive mechanism to move the substrate while keeping airtight between intermediate chamber 4m and each of chambers 4n, 4i1 to 4l4 and 4p, and besides, the number of chambers which can be provided around intermediate chamber 4m is spatially limited, so the method is not much employed in practice.
If a silicon-based thin film photoelectric converter includes a photoelectric conversion unit including a pin or nip junction, a polycrystalline silicon-based thin film or a microcrystalline silicon-based thin film partially having an amorphous state would be preferably used for a conductivity type layer such as a p- or n-layer. When such a crystalline conductivity type layer is formed by plasma CVD, an electrode material layer such as a transparent conductive oxide film or metallic film serves as an underlying layer for a conductivity type layer to be deposited before a photoelectric conversion layer is deposited and the photoelectric conversion layer serves as an underlying layer for a conductivity type layer to be deposited after the photoelectric conversion layer is deposited,. The conductivity type layer would be preferably deposited under plasma CVD conditions to introduce high discharge power using a material gas diluted with hydrogen in order to improve the function of the layer. However, if a conductive type microcrystalline silicon-based film is deposited under active plasma conditions, high energy plasma damages the underlying layer or the mutual diffusion between the impurity atoms becomes active to deteriorate the interface characteristic of semiconductor junction, so that grains cannot grow enough in the conductivity type layers and an incomplete microcrystalline film may result. The speed of film deposition is extremely low by the conventional plasma CVD, which impedes the throughput in the manufacture of photoelectric converters from increasing and the cost from being reduced.
In view of the above described disadvantages associated with the conventional techniques, it is an object of the present invention to improve the throughput in the manufacturing process by increasing the forming speed of a crystalline silicon-based photoelectric conversion layer formed by low temperature plasma CVD, to improve the performance of a photoelectric converter and to provide a plasma CVD apparatus preferably used for these purposes.
In a method of manufacturing a silicon-based thin film photoelectric converter according to one embodiment of the present invention, the photoelectric converter includes one or more photoelectric conversion units formed on a substrate, at least one of the photoelectric conversion units is a polycrystalline photoelectric conversion unit including a semiconductor layer of one conductivity type, a crystalline silicon-based thin film photoelectric conversion layer, and a semiconductor layer of the opposite conductivity type, sequentially deposited upon one another, the crystalline photoelectric conversion layer is deposited under the plasma CVD conditions: the underlying layer is at a temperature of 550xc2x0 C. or less; the gas introduced into a plasma reaction chamber has a silane-based gas and a hydrogen gas as main constituents; the flow rate of the hydrogen gas is 50 times or more that of the silane-based gas; the pressure in the plasma reaction chamber is set at 3 Torr or more; and the forming speed of the film is not less than 17 nm/min in the thickness-wise direction.
A method of manufacturing a silicon-based thin film photoelectric converter by plasma CVD according to another embodiment of the present invention is characterized in that an n-type layer is formed to have a thickness in the range from 2 to 50 nm, an i-type crystalline silicon-based photoelectric conversion layer is formed to have a thickness in the range from 1 to 5 xcexcm at a deposition speed of 17 nm/min or more, a p-type layer is formed to have a thickness in the range from 2 to 50 nm, and these n-type layer, i-type photoelectric conversion layer, and p-type layer are successively formed in the same plasma CVD reaction chamber.
In a method of manufacturing a silicon-based thin film photoelectric converter according to another embodiment of the present invention, the photoelectric converter includes one or more photoelectric conversion units formed on a substrate, and at least one of the photoelectric conversion units is a polycrystalline photoelectric conversion unit including a semiconductor layer of one conductivity type, a crystalline silicon-based photoelectric conversion layer of substantially intrinsic semiconductor, and a semiconductor layer of the opposite conductivity type, sequentially layered upon one another by plasma CVD, at least one of the conductivity type layers includes a crystalline silicon-based thin film, and the crystalline conductivity type layer is deposited by plasma CVD under the following conditions: the underlying layer is at 550xc2x0 C. or less; a material gas introduced into the plasma reaction chamber has a silane-based gas and a diluted gas containing hydrogen as main constituents and the flow rate of the diluted gas is 100 times or more that of the silane-based gas; the pressure in the plasma reaction chamber is set at 5 Torr or more; and a film is formed at a speed of 12 nm/min or more in the thickness-wise direction to have a thickness in the range from 2 to 50 nm.